Jeffries, who was previously head of digital at Edelman until March this year, has been appointed as client partner, joining the Havas agency's management team alongside MD Matt Buchanan and founder Kat Thomas.
Her focus includes strategic consultation for clients across digital and social media, as well as a leadership role on the agency's Nokia Mobile account, which it picked up earlier this year. She joined the agency last month.
Jeffries will also be responsible for the UK rollout of One Green Bean's proprietary influencer collaboration product, Co-maker. It has already been used by clients Costa and Nestlé, according to the agency.
Meanwhile, the agency has appointed Warr as associate creative director. His remit will include offering creative direction, counsel and support for all clients.
Before joining One Green Bean this month, Warr was associate director at Stripe Communications, his LinkedIn profile states. Prior to this he held a number of roles at Edelman, including as a creative between 2015 and 2016, and as an account director before this.
He was also head of consumer technology at Mischief between 2011 and 2013.
Both Jeffries and Warr will report to Buchanan, who said: "Our growth in the last six months has been driven by both existing clients and the win of several pitches. It's an exciting time for us, and the arrival of Cath and Chris will see us broaden our shoulders significantly."